Temperature dependence of ultraviolet line parameters in network and internetwork regions of the quiet Sun and coronal holes
1 School of Earth and Space Sciences, Peking University, 100871 Beijing, PR China
2 High Altitude Observatory, National Center for Atmospheric Research, PO Box 3000, Boulder, CO 80307, USA
3 Max-Planck-Institut für Sonnensystemforschung, 37191 Katlenburg-Lindau, Germany
4 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
5 School of Space Science and Physics, Shandong University at Weihai, 264209 Weihai, PR China
Received: 10 August 2012
Accepted: 22 July 2013
Aims. We study the temperature dependence of the average Doppler shift and the non-thermal line width in network and internetwork regions for both the quiet Sun (QS) and the coronal hole (CH), by using observations of the Solar Ultraviolet Measurements of Emitted Radiation instrument onboard the Solar and Heliospheric Observatory spacecraft.
Methods. We obtain the average Doppler shift and non-thermal line width in the network regions of QS, internetwork regions of QS, network regions of CH, and internetwork regions of CH by applying a single-Gaussian fit to the line profiles averaged in each of the four regions. The formation temperatures of the lines we use cover the range from 104 to 1.2 × 106 K. Two simple scenarios are proposed to explain the temperature dependence of the line parameters in the network regions. In one of the scenarios, the spectral line consists of three components: a rapid, weak upflow generated in the lower atmosphere, a nearly static background, and a slow cooling downflow. In the other scenario, there are just two components, which include a bright core component and a faint wide tail one.
Results. An enhancement of the Doppler shift magnitude and the non-thermal line width in network regions compared to the internetwork regions is reported. We also report that most transition region lines are less redshifted (by 0−8 km s-1) and broader (by 0−5 km s-1) in CH compared to the counterparts of QS. In internetwork regions, the difference in the Doppler shifts between the coronal hole and the QS is slightly smaller, especially for the lines with formation temperatures lower than 2 × 105 K. And the two simple scenarios can reproduce the variation in the line parameters with the temperature very well.
Conclusions. Our results suggest that the physical processes in network and internetwork regions are different and that one needs to separate network and internetwork when discussing dynamics and physical properties of the solar atmosphere. The agreement between the results of the observation and our scenarios suggests that the temperature dependence of Doppler shifts and line widths might be caused by the different relative contributions of the three components at different temperatures. The results may shed new light on our understanding of the complex chromosphere-corona mass cycle. However, the existing observational results do not allow us to distinguish between the two scenarios. At this stage, a high-resolution instrument Interface Region Imaging Spectrograph is highly desirable.
Key words: Sun: atmosphere / Sun: UV radiation / line: profiles
© ESO, 2013